Various types of liquid crystal cells and displays which use liquid crystal cells are known. Examples of liquid crystal cells are those known as twisted nematic, super twist, and variable birefringence. In a typical liquid crystal cell, liquid crystal material is contained between a pair of substrates, and a seal is provided about the edge of the liquid crystal cell between the substrates to prevent leakage of the liquid crystal material. Exemplary substrates are glass. Usually some type of surface treatment is provided the surface of one or both substrates confronting or bounding the liquid crystal material for the purpose of providing a preferred alignment of the liquid crystal material, and especially to obtain such alignment in the absence of the energization of the liquid crystal cell. An example of such energization is that provided by an electric field derived by applying a voltage across a pair of electrodes located on the respective confronting substrates.
Liquid crystal alignment usually refers to the alignment of the liquid crystal structure, especially when referring to nematic liquid crystal material. Alignment also may refer to the direction of the liquid crystal director, structural organization of the liquid crystal material, etc., as is well known in the art. Several of the types of liquid crystal material include nematic liquid crystal, smectic liquid crystal and cholesteric liquid crystal. Nematic liquid crystal tends to align directionally with respect to a surface of a liquid crystal cell with which the liquid crystal is in relative proximity (meaning next to or near) or is directly engaged, etc.
The surface treatment referred to above tends to cause that liquid crystal material which is generally in proximity to the particular substrate to align in a preferred direction. Examples of surface treatment include rubbing the surface with cotton, felt, or some other material in a particular direction, which causes the liquid crystal material to align relative to that direction. Another example of surface treatment includes applying a silicon oxide (SiO) coating to the surface using an evaporation technique; depending on the angle of evaporation relative to the substrate surface and other factors, which are known in the art, the liquid crystal material will tend to align in a particular direction, which usually is generally parallel to the surface but at some tilt angle, as is well known. Other examples of surface treatment includes the applying of a polyvinyl alcohol (PVA) material to the surface or a polyimide coating to the surface. The various coatings also may be rubbed using cotton, felt or some other material to provide the desired alignment characteristics.
In a twisted nematic liquid crystal cell the rub direction or primary alignment direction (not considering the tilt angle) at one substrate is at an angle other than 0 degrees or 180 degrees relative to the rub direction or primary alignment direction of the other substrate. For example, the rub or alignment directions of respective substrates may be at 90 degrees to each other in the standard twisted nematic liquid crystal cell so as to provide for a helical twist of 90 degrees in the liquid crystal alignment direction from one substrate surface to the other. In an example of a variable birefringence liquid crystal cell, the rub direction or primary alignment direction of one substrate usually is oriented in parallel with the rub direction or primary alignment direction of the other substrate. The mentioned parallel alignment may be at 0 degrees or at 180 degrees, and there may be the same or different tilt angles at or near respective substrate surfaces of the cell. Other relative alignment directions may be used in respective liquid crystal cells, as is known.
Most current displays in head mounted display systems utilize the twisted nematic mode of operation of the liquid crystal cell(s) used in such displays. The speed of this type of display is relatively slow causing the image to tend to smear when rapidly changing video is displayed. The speed of response (switching speed from one light transmissive condition to the other, for example) is too slow to allow color images to be created by color sequential addressing also referred to as field sequential or frame sequential operation. It would be desirable to provide increased switching speed of operation in a liquid crystal display device. The switching speed of at least some variable birefringence liquid crystal cells has been found to be faster than that of twisted nematic liquid crystal cells. (See, for example, U.S. Pat. Nos. 4,385,806, 4,436,376, 4,540,243, Re. 32,521, and 4,582,396.)
An example of field sequential or frame sequential operation, sometimes also referred to as color sequential addressing, is described in U.S. Pat. No. 4,582,396. A liquid crystal display sequentially presents respective images at a speed faster than the human eye can separately distinguish or follow. The sequential images are separated in time rather than in space. The images are merged or integrated by the eye to compose an image which in effect is a combination of several sequential images. In this way images having multiple colors can be produced. For example, in the mentioned patent using a series of color filters that are responsive to the direction of plane polarized light a multicolor image can be produced; at one moment in time an image is produced and is filtered by one color filter; and at a subsequent moment in time the same or a correlated image is produced and is filtered by a second color filter; the images are combined (integrated) by the viewer's eye so that a combined image is seen.
In the absence of a prescribed input, such as an electric field, to the liquid crystal cell, the alignment of liquid crystal material usually is influenced by the surface and surface treatment, especially for nematic liquid crystal or operationally nematic liquid crystal. Operationally nematic means the liquid crystal operates sufficiently similarly to nematic liquid crystal as to be useful in the present invention described below. However, upon application of an electric field, at least some of the liquid crystal material tends to align with respect to the field, which tends to overcome the influence of the surfaces on liquid crystal alignment. The stronger the electric field, i.e., the greater the magnitude of the field or the voltage causing the field, the greater the amount of the liquid crystal material that tends to align with respect to the field and/or the more accurately the liquid crystal aligns with the field. The electric field may be developed by applying a voltage between an electrode located on one of the substrates and an electrode on the other of the substrates. In some liquid crystal cells known as active matrix or thin film transistor (TFT) devices, a number of electronic components, such as transistors, capacitors, etc., may be provided at or on one of the substrates or surfaces thereof to develop the appropriate electrical energization for liquid crystal material in the liquid crystal cell at one or more locations in the cell.
In an exemplary flat panel display technology an active matrix liquid crystal display is fabricated from substrates of amorphous or polysilicon thin film transistor arrays deposited on quartz or glass. Displays of this type are typically back lit and viewed in transmission. They suffer from several disadvantages. First, to obtain a colored image filters are deposited at each pixel (sometimes referred to in the art as a picture element where a portion of an image is created; the sum of all or many of the pixels of the display can be used to create a total image, as is well known). The fabrication of such filters is a difficult and expensive process and results in a display with a dramatically reduced light transmission as well as 1/3 reduction in image resolution since three filters (e.g., red, green and blue) are required at separate areas of each pixel. Second, the thin film transistors of such arrays must be physically large causing a large fraction of each pixel to be non-functional optically, which reduces the amount of light output capability for each pixel. Also, since the traces of such arrays are opaque and black in transmission, the inter pixel spaces are emphasized.
An alternate approach addressing the above deficiencies of the substrates described above has been to fabricate a single crystal silicon array on a wafer. This allows the use of conventional semiconductor processing, and the transistors can be physically smaller than those mentioned above. However, the array must be "lifted" from the wafer and deposited onto the glass substrate. The process to do this introduces additional steps to the wafer fabrication process and adds to the cost of the substrate.
As is described further below, the present invention relates to liquid crystal cells and displays which are fabricated directly on the semiconductor wafer substrate.
A disadvantage of an active matrix substrate used in a liquid crystal cell is the non-uniformity of the surface thereof, which usually has various peaks and valleys in the surface due to the electronic components formed therein. Such surface non-uniformity may have a noticeable degrading effect on the quality of images produced by a liquid crystal cell. For example, a change in path length of light in such a liquid crystal cell or a random alignment or misalignment of liquid crystal material due to a peak or a valley in a substrate may uncontrollably change optical phase retardation. This negative impact on the display is compounded if the display is used in a reflective mode because light then transmits through the liquid crystal twice.
To increase the contrast, resolution, and brightness of images created by a liquid crystal cell in a display and to facilitate manufacturing, it would be desirable to use an active matrix drive device for the liquid crystal cell. In the present invention these advantages can be accomplished, for example, by using a display operated in reflective mode. The active matrix transistors then can be located in the substrate beneath the electrodes of the matrix array, if desired, which increases the optical operational area of each pixel. In contrast, in a transmission display the respective active matrix transistors block light in part of each pixel. However, the fact that the surface of an active matrix substrate is rough or unsmooth, which disrupts a uniform liquid crystal alignment, has lead away from using such a substrate, especially in a variable birefringence liquid crystal cell.
Homogeneous alignment usually refers to an alignment of liquid crystal material in a direction that is generally parallel to the plane of a surface of a substrate of a liquid crystal cell. The liquid crystal material may have a tilt angle relative to the surface. Various tilt angles are possible and are used in various types of liquid crystal devices, as is well known. Exemplary liquid crystal cells which use homogeneous alignment are twisted nematic liquid crystal cells, used, for example, in watch and computer displays. Homeotropic alignment of liquid crystal material usually refers to an alignment that is generally perpendicular (sometimes referred to as normal) to the surface or the plane of the surface of the substrate of the liquid crystal cell. In the past homeotropic alignment was used in liquid crystal cells that work in a dynamic scattering mode in response to application of an electric current.
Nematic liquid crystal and smectic liquid crystal can have characteristics of birefringence, whereby the ordinary index of refraction and the extraordinary index of refraction are different. In a variable birefringence liquid crystal cell, by changing the orientation or alignment of the liquid crystal (or some of the liquid crystal) relative to the direction of light propagating through the liquid crystal, optical phase retardation can be varied correspondingly. Examples of variable birefringence liquid crystal cells in which optical phase retardation can be varied are described in U.S. Pat. Nos. 4,385,806, 4,436,376, 4,540,243, Re. 32,521, and 4,582,396, which are incorporated by reference. As is described in those patents, by changing the applied input, such as electric field, the alignment of liquid crystal material in the liquid crystal cells can be altered thereby to alter the effective optical phase retardation of the light transmitted through the liquid crystal material. As also is described in the just-mentioned patents, the liquid crystal material in proximity to the respective substrates has generally homogeneous alignment; these portions of the liquid crystal material or liquid crystal layer sometimes are referred to as the surface layers of the liquid crystal material and it is these layers or at least parts thereof which switch alignment in response to applied field input during operation of the liquid crystal cell to change the optical phase characteristics of the liquid crystal cell in response to say application or removal of electric field. The surface layers or surface portions are separated by a portion of the liquid crystal material or a layer thereof which generally is aligned perpendicularly with respect to the surfaces. Such perpendicularly aligned liquid crystal material tends not to contribute to optical phase retardation (or whatever contribution it has is relatively minimal compared to the possible phase retardation provided by the surface portions). Such generally perpendicularly aligned liquid crystal material also may tend to separate the physical/mechanical interaction of the two surface portions of liquid crystal material during operation of the liquid crystal cell as the surface portions switch from one alignment to the other. The liquid crystal material which tends to separate the surface portions sometimes is referred to as the "bulk" liquid crystal; whether the bulk is more or less quantity of liquid crystal than the surface portions does not deter use of such label "bulk". Various means may be used to align the bulk portion of the liquid crystal material. Those means may be electrical, mechanical, a combination thereof, or some other means, for example, as is described in the aforementioned patents.
It would be desirable to provide variable optical phase retardation capability in a reflective liquid crystal cell and display using such a cell, and, especially, to do so in a liquid crystal cell that has an active matrix type substrate. It would be desirable to provide substantial uniformity of operation and optical phase retardation characteristics in a variable birefringence liquid crystal cell while reducing the affect of and/or without regard to disparities in cell thickness due to peaks and valleys in the substrate.